Add VM and compiler support for calls to known procedures

Add VM and compiler support for calls to known procedures

* module/language/cps.scm ($callk): New expression type, for calls to
  known labels.  Part of "low CPS".
* module/language/cps/arities.scm:
* module/language/cps/closure-conversion.scm:
* module/language/cps/compile-bytecode.scm:
* module/language/cps/dce.scm:
* module/language/cps/dfg.scm:
* module/language/cps/effects-analysis.scm:
* module/language/cps/simplify.scm:
* module/language/cps/slot-allocation.scm:
* module/language/cps/verify.scm: Adapt call sites.

* libguile/vm-engine.c (call-label, tail-call-label): New instructions.
  Renumber the rest; this is an ABI change.

* libguile/_scm.h (SCM_OBJCODE_MINOR_VERSION):
* module/system/vm/assembler.scm (*bytecode-minor-version*): Bump.

* doc/ref/compiler.texi (CPS in Guile): Document $callk.